PhD Oral Exam - Venkata Ratnam Vakacharla, Electrical and Computer Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

This thesis studies and analyses the suitability of resonant tanks in load resonant current-fed dc/dc converters to improve the soft-switching range of the input voltage and lad current while achieving high gain, high power density, and high efficiency. It also investigates upon the light load efficiency improvements. In traditional voltage-fed resonant converters, it is observed that the converter characteristics can be fine-tuned to design the requirements by proper selection of resonant tank. In addition, the resonant tank can integrate the transformer non-idealities and circuit/device parasitic in circuit operation thereby suppressing the consequent voltage spikes across the semiconductor devices. Since voltage-fed converters is fundamentally not suitable for high gain and low voltage applications, this thesis attempts to improve current-fed dc/dc converter characteristics with resonant tanks.

In this thesis, a current-fed load resonant DC/DC converter topology is proposed whose characteristics are tuneable with the adopted resonant tank. Further, this thesis proposes a simple technique to ease and improve accuracy of the Fundamental Harmonic Analysis (FHA), which would have been complex otherwise due to capacitive termination of proposed converter. Initially, the characteristics of the proposed converter topology with a parallel resonance derived LCC-T resonant tank is studied to implement zero voltage switching (ZVS) and zero current switching (ZCS) of the semiconductor devices. Three-phase topology of the same has been investigated and analysed. Following the study and a need to further improve the characteristics of resonant dc/dc converter, a series resonance based LCL resonant converter, a dual of the parallel resonance tank is studied and analysed. The load resonant converters are redeemed for integration of PV/fuel cells. Further, for high power applications, suitability of load resonant converters is verified by adopting resonant tank in three-phase topology. Proof-of-concept hardware prototypes are designed and developed in the laboratory to demonstrate the performance and the merits of the proposed soft-switching resonant converter topologies as well as to prove the proposed theory and the claims.